I am now considering replacing my system hard drive with a SSD drive, and I think it'd be a good idea to do a clean install at the same time. But like I said, I don't have an install disc or even an ISO file for Windows 8, so I'm a bit at a loss about how to proceed. I searched the web and the W8 official site without finding much success.

What are my options? Is it possible to download W8 it from Microsoft (I searched but found nothing), or do I have to reinstall Windows 7 (for which I have the disc) and then re-do the upgrade process? Or something else?

You should run it on a Windows 7 PC (e.g. friends PC). Then select "install by creating media". It will create an ISO file. Burn to DVD and make clean install. After installing try to activate the system. If you can not activate (error code: 0xc004f061) then call the Microsoft Customer Service. They will help you activating the system (you will need Windows 7 Product Key and the new Windows 8 Product Key).

Thanks for the link and the explanation. Apparently you don't need to run the Upgrade Assistant from Win7: you can also run it from Win8, i.e. my present PC (which is simpler). I'm burning it right now on an USB flash drive. I'll see how it works and if I can install W8 directly without first installing W7.
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s427Jan 26 '13 at 18:16